Embodiments of the invention relate generally to electronic power conversion and, more particularly, to a photovoltaic system and method of controlling thereof that monitors and regulates the output voltage of a photovoltaic array to be within a desired voltage range for operating the photovoltaic inverter.
Photovoltaic (PV) power systems are power systems that employ a plurality of solar modules to convert sunlight into electricity. PV systems include multiple components, including PV modules, mechanical and electrical connections and mountings, and means of regulating or modifying the electrical output. One common arrangement in PV systems is for several PV modules to be connected in series to form a PV string, with multiple PV strings in a PV system then being combined in parallel to aggregate the current in a PV array.
In operation, PV modules generate direct current (DC) power, with the level of DC current being dependent on solar radiation and the level of DC voltage dependent on temperature. When alternating current (AC) power is desired, an inverter is used to convert the DC energy into AC energy, such as AC energy suitable for transfer to a power grid.
For converting the varying DC voltage of a PV array to the fixed frequency AC voltage of the power grid, PV inverters may employ a single-stage conversion power circuit in which a transformer is employed to boost the AC voltage. Alternately, PV inverters may employ a two-stage conversion power circuit that uses a DC link as an intermediate energy storage step, which means that the converter first converts the unstable PV array voltage to a stable DC voltage. In a two-stage PV inverter, often the first stage includes a boost converter, and the second stage includes a single-phase or three-phase inverter system. The PV inverter then subsequently converts the stable voltage into an AC current that can be injected into the grid.
The open voltage (Voc) and maximum power (Pmax) of a photovoltaic cell is dependent upon the ambient temperature. In particular, Voc and Pmax increase as the ambient temperature decreases from 25 degrees Celsius. Accordingly, the voltage-power curve of a PV array panel varies in both power and voltage as the ambient temperature of the PV array decreases.
If the PV array voltage is below a threshold, the boost converter operates to boost the PV voltage output from the PV array. If the PV array voltage is above the threshold, the boost circuit will stop switching and allow the PV voltage to pass through the diode to the bus capacitors and DC/AC circuit. As the PV array voltage continues to increase above the threshold, the operating temperature of the PV inverter also increases. Due to the high temperature and voltage protection integrated into the PV inverter, the PV inverter enters a voltage protection mode in which it regulates its output power and/or shuts down inverter operation in order to prevent the inverter from overheating or failing. While the voltage protection mode extends the life of the PV inverter, it also negatively affects the operating efficiency of the PV inverter.
It would therefore be desirable to provide a PV system wherein the PV inverter would be capable of operating at high PV array voltages without entering a voltage protection mode.